金属间化合物DyMn2Ge2的自发磁相变和场诱导的磁相变

在10—800K的温度范围内用X射线衍射方法测量了DyMn₂Ge₂化合物的晶格常数与温度的变化关系,观察到高温时DyMn₂Ge₂由顺磁状态到反铁磁状态的自发磁相变伴随着晶格常数a的负的磁弹性异常现象.在4.2K—200K的温度范围内测量了DyMn₂Ge₂的交流磁化率.在交换相互作用的分子场模型近似下,从理论上分析讨论了DyMn₂Ge₂的低温自发磁相变和场诱导的磁相变.计算了DyMn₂Ge₂单晶的磁化强度与温度的变化关系以及不同温度下外磁场沿晶轴c方向时的磁化曲线.理论分析和计算结果表明,温度低于33K时在DyMn₂Ge₂中观察到的场诱导的一级磁相变为由亚铁磁状态(Fi)到中间态(IS)相变. 关键词： 稀土-过渡族金属间化合物 磁结构 磁相变     

Magnetic phase diagram of GdNi2Ge2: a magnetisation and specific-heat study

The antiferromagnetic body-centred tetragonal compound GdNi₂Ge₂ orders at 28 K. Successive magnetic phase transitions are observed by specific-heat and magnetisation measurements as a function of temperature in different applied magnetic fields. Plots of M² vs. B/M (Arrott-plots) show various anomalies. On the basis of the experimental results, a magnetic phase diagram is constructed. The multiple magnetic phase transitions are discussed in terms of competing ordering modes in the Gd sublattice.     

Spiral spin structures of RMn6Ge6 (R=Gd, Tb, Dy)

The field dependences of the magnetic moment of several Gd_XY_1−XMn₆Ge₆ compounds have been analysed by a mean-field theory of several spin structures. The assumption of spiral spin structures at low temperatures in these compounds has been confirmed. In addition to this, the magnetic phase diagrams of TbMn₆Ge₆ and DyMn₆Ge₆ have been deduced from magnetisation measurements of powder samples. In contrast to GdMn₆Ge₆, the crystal fields of these compounds produce a variety of additional spin phases.     

Induced magnetic phase transitions in rare-earth intermetallic compounds RMn2Ge2 in ultrastrong magnetic fields

The differential magnetic susceptibility of intermetallic compounds RMn₂Ge₂ (R=Gd, Tb, Dy, Ho, Y) with a layered tetragonal structure is measured in pulsed magnetic fields up to 130 T. It is found that all these compounds undergo a first-order magnetic phase transition in strong magnetic fields. The nature of this transition is discussed, and it is found that a change in the magnetic state of the manganese sublattice is responsible for the transition.     

Magnetic properties of the HoMn2Ge2 intermetallic compound

The magnetic properties of a tetragonal intermetallic compound, namely, HoMn₂Ge₂, are investigated experimentally and theoretically. The experimental temperature dependences of the initial magnetic susceptibility and the lattice parameters are obtained in alternating and static magnetic fields. The magnetization curves are measured in strong magnetic fields up to 50 T. The parameters of the crystal field and Ho-Mn and Mn-Mn exchange interactions are determined, and the temperature dependence of the magnetic field of the phase transition from an antiferromagnetic phase to a ferromagnetic phase in a magnetic field aligned along the tetragonal axis is calculated.     

Mössbauer investigation of magnetic phase transitions in FeI2

Mössbauer spectra were measured on single crystals of FeI₂ at temperatures between 1.5 and 40°K in magnetic fields up to 6.5 T. The measurements show that the recently discovered magnetic phase transitions on FeI₂ are characterized by changes in the ratio of spins parallel and antiparallel to the c-axis. The five magnetic phases have the following ratio of parallel to anti-parallel spins: 6 : 6, 8 : 4, 9 : 3, 10 : 2 and 12 : 0.     

Ferromagnetism modulation by phase change in Mn-doped GeTe chalcogenide magnetic materials

In this work, an effective method to modulate the ferromagnetic properties of Mn-doped GeTe chalcogenide-based phase change materials is presented. The microstructure of the phase change magnetic material Ge_1?x Mn _x Te thin films was studied. The X-ray diffraction results demonstrate that the as-deposited films are amorphous, and the crystalline films are formed after annealing at 350 °C for 10 min. Crystallographic structure investigation shows the existence of some secondary magnetic phases. The lattice parameters of Ge_1?x Mn _x Te (x = 0.04, 0.12 and 0.15) thin films are found to be slightly different with changes of Mn compositions. The structural analysis clearly indicates that all the films have a stable rhombohedral face-centered cubic polycrystalline structure. The magnetic properties of the amorphous and crystalline Ge_0.96Mn_0.04Te were investigated. The measurements of magnetization (M) as a function of the magnetic field (H) show that both amorphous and crystalline phases of Ge_0.96Mn_0.04Te thin film are ferromagnetic and there is drastic variation between amorphous and crystalline states. The temperature (T) dependence of magnetizations at zero field cooling (ZFC) and field cooling (FC) conditions of the crystalline Ge_0.96Mn_0.04Te thin film under different applied magnetic fields were performed. The measured data at 100 and 300 Oe applied magnetic fields show large bifurcations in the ZFC and FC curves while on the 5,000 Oe magnetic field there is no deviation.     

The influence of magnetoelastic interaction on structural phase transitions in cubic ferromagnetics

In the framework of the Landau theory of phase transitions, the influence of the magnetoelastic interaction on structural transitions in cubic ferromagnetics with a positive first magnetic anisotropy constant is analyzed. It is shown that structural transitions are not accompanied by a reorientation of magnetization in this case. The phase diagrams of such ferromagnetics either contain a termination point of the structural transition or a critical point in which the first-order transition is replaced by a second-order one. Magnetoelastic interaction also leads to the appearance of an interval of the ferromagnetic parameters in which a coupled first-order structural-magnetic transition exists. The phase T?x diagram for Heusler Ni_2+x Mn_1?x Ga alloys is calculated, which is in good agreement with the experimental phase diagram of these alloys.     

Magnetism influenced by structural disorder in melt-spun DyMn6 − x Ge6 − x Fe x Al x (x = 2.5, 3)

Different chemical and/or geometrical orders were found in melt-spun DyMn_6???x Ge_6???x Fe _x Al _x with x = 2.5 and 3 having fully amorphous and mixed (crystalline and amorphous) structure, respectively. Thermal variations in magnetization M from liquid helium up to room temperature for both samples are similar. Magnetization value at zero field cooled curve reaches about 0.1 μ_B per formula unit at 2 K and then increases. Two maxima are visible, the first at 50 K (a sharp effect) and the second very broad ranging from 150 to 200 K. ⁵⁷Fe Mössbauer spectrometry investigation revealed a remaining magnetic component in addition to a prevailing quadrupolar feature. Application of a weak external magnetic field causes an increase in the mean hyperfine magnetic field B _hyp and the volume fraction of magnetic component. This observation was confirmed by results of M(T), M(H) and AC magnetic susceptibility measurements. In short-range ordered crystallographic zones characteristic of melt-spun DyMn_6???x Ge_6???x Fe _x Al _x (x = 2.5, 3) alloys, the related magnetic ordering, called the mictomagnetism or the cluster spin glass appears.     

Magnetic phase transition and the corresponding magnetostriction of intermetallic compounds RMnsGe2（R=Sm,Gd）

The temperature dependence of lattice constants a and c of intermetallic compounds RMn₂Ge₂ (R=Sm, Gd) is measured in the temperature range 10－800K by using the x-ray diffraction method. The magnetoelastic anomalies of lattice constants are found at the different kinds of spontaneous magnetic transitions. The transversal and longitudinal magnetostrictions of polycrystalline samples are measured in the pulse magnetic field up to 25T. In the external magnetic field there occurs a first-order field-induced antiferromagnetism－ferromagnetism transition in the Mn sublattice, which gives rise to a large magnetostriction. The magnitude of magnetostrictions is as large as 10^-3. The transversal and longitudinal magnetostrictions have the same sign and are almost equal. This indicates that the magnetostriction is isotropic and mainly caused by the interlayer Mn－Mn exchange interaction. The experimental results are explained in the framework of a two-sublattice ferrimagnet with the negative exchange interaction in one of the sublattices by taking into account the lattice constant dependence of interlayer Mn－Mn exchange interaction.     

Magnetism in PrIr2Si2: A single crystal study

We present the results of magnetization, susceptibility and specific-heat measurements of the high-temperature (HT) and low-temperature (LT) phases of PrIr₂Si₂ performed on single-crystalline samples. The HT and LT phases adopt the tetragonal CaBe₂Ge₂-type and ThCr₂Si₂-type structure, respectively. We have found no magnetic phase transition for the HT phase at temperatures down to 2 K. On the other hand, the LT phase apparently orders antiferromagnetically (AF) at 45.5 K and undergoes a transition to another AF phase at T_t=23.7 K. Complexity of the magnetic phase diagram is amplified by two metamagnetic transitions induced by magnetic field applied along the c-axis at temperatures below T_t. The results will be discussed with respect to other polymorphic compounds PrNi₂As₂ and UCo₂Ge₂.     

Magnetic excitations of HoAl2 in a magnetic field

The ground state spin wave excitation energies of single crystalline HoAl₂ have been studied at T= 4.2 K in external magnetic fields up to 7 T by means of inelastic neutron scattering. The results have been interpreted in terms of a cubic crystalline electric field using the parameters determined by magnetization measurements and an exchange interaction with the exchange parameters taken from the zero field measurements.     

Magnetic and superconducting phase diagrams in ErNi2B2C

We present measurements of the superconducting upper critical field H_c2(T) and the magnetic phase diagram of the superconductor ErNi₂B₂C made with a scanning tunneling microscope (STM). The magnetic field was applied in the basal plane of the tetragonal crystal structure. We have found large gapless regions in the superconducting phase diagram of ErNi₂B₂C, extending between different magnetic transitions. A close correlation between magnetic transitions and H_c2(T) is found, showing that superconductivity is strongly linked to magnetism.     

Magnetocaloric effect in high-energy ball-milled Gd5Si2Ge2 and Gd5Si2Ge2/Fe nanopowders

Evolution of structure and magnetocaloric properties in ball-milled Gd₅Si₂Ge₂ and Gd₅Si₂Ge₂/0.1 wt% Fe nanostructured powders were investigated. The high-energy ball-milled powders were composed of very fine grains (70–80 nm). Magnetization decreased with milling time due to decrease in the grain size and randomization of the magnetic moments at the surface. The magnetic entropy change (ΔS_M) was calculated from the isothermal magnetization curves and a maximum value of 0.45 J/kg K was obtained for 32 h milled Gd₅Si₂Ge₂ alloy powder for a magnetic field change of 2 T while it was still low in Fe-contained alloy powders. The thermo-magnetic measurements revealed that the milled powders display distribution of magnetic transitions, which is desirable for practical magnetic refrigerant to cover a wide temperature span.     

Reversible structural phase transition in Ni-Mn-Ga alloys in a magnetic field

In Ni_2+x Mn_1?x Ga shape-memory ferromagnetic alloys with coincident magnetic and structural phase transitions, a reversible structural field-induced phase transition was observed at constant temperature and pressure in magnetic fields of about 10 T. Computational results are in qualitative agreement with experiment.     

Magnetic properties of ternary RMn2Si2 and RMn2Ge2 compounds

Magnetic properties of RMn₂Si₂ and RMn₂Ge₂ compounds, where R is a rare earth metal, have been investigated by magnetometric measurements. RMn₂Ge₂ (where R is a light rare earth) and LaMn₂Si₂ are ferromagnets. Remaining compounds have antiferromagnetic properties. DyMn₂Si₂ and ErMn₂Si₂ show ferromagnetic properties at low temperatures. It was confirmed that the value of Curie (or Néel) temperature for the Mn sublattice decreases with increasing c constant.     

Magnetic anisotropy and magnetostriction of Lu2Fe17 single crystal

The magnetic properties of Lu₂Fe₁₇ single crystal have been studied by means of magnetization, susceptibility and magnetostriction measurements. The unusual magnetic behavior with two magnetic phase transitions has been observed in magnetic fields up to 50 Oe. The magnetostriction of the Lu₂Fe₁₇ compound has the maximum at temperature T≈285 K at which the paraprocess makes the main contribution to the magnetization.     

X-ray studies of magnetic phase transitions in the intermetallic compounds RMn2Ge2 (R=La, Sm, Gd, Nd, Tb, and Y)

The lattice parameters a and c of the tetragonal intermetallic compounds RMn₂Ge₂ (R=La, Sm, Gd, Nd, Tb, and Y) have been measured by x-ray diffraction in the temperature interval 10–800 K. Anomalies are observed in the temperature dependence of a and c due to phase transitions from the paramagnetic to the magnetically ordered state in the Mn subsystem, transitions between various magnetically ordered phases due to a change in the magnitude and sign of the Mn-Mn exchange interaction, and magnetic transitions caused by ordering of the rare-earth subsystem leading to a rearrangement of the magnetic structure of the Mn subsystem. It is concluded that, along with the lattice parameter a, the lattice parameter c also has an influence on the Mn-Mn exchange interaction. Fiz. Tverd. Tela (St. Petersburg) 41, 2053–2058 (November 1999)     

Differential thermal analysis of the field induced phase transitions of (TMTSF)2ClO4 above 1.2 K

We present some results of a differential thermal analysis of the magnetic field induced phase transitions in the organic conductor (TMTSF)₂ClO₄ above 1.2 K. This study shows that transitions between different spin density wave states are first order and that the total entropy change involved in the two detected transitions (in the temperature range 1.2–2 K) is close to that of the quasi-one-dimensional electron gas. Above 2 and 4.2 K, only a single transition has been detected in our measurements. The entropy of that transition decreases and extrapolates to zero near 5 K. We present some arguments suggesting that if longitudinal nesting (2k_F, 0, 0) is to take place in the semi-metallic SDW phase at high fields it exists only above 2 K or so.     

A magnetic study of the ThCr2Si2-type RPd2Ge2 (R=Pr, Nd) compounds. Magnetic structure of PrPd2Ge2 from powder neutron diffraction

The magnetic properties of the PrPd₂Ge₂ and NdPd₂Ge₂ compounds have been investigated by magnetic measurements, specific heat measurements and neutron diffraction experiments. The PrPd₂Ge₂ compound orders antiferromagnetically below T_N=5.0(2) with an original modulated magnetic structure characterized by a magnetic cell three times larger than the chemical one by tripling of the c parameter. The palladium atom is non magnetic and the Pr moments are parallel to the c-axis with a value of ≈2.0 μ_B at 2 K. The specific heat measurements clearly detect a low temperature transition for the NdPd₂Ge₂ compound, interpreted as a Nd sublattice antiferromagnetic ordering below 1.3(2) K.